1. Field of the Invention
This invention lies in the field of linings for large diameter boreholes drilled into the earth.
More particularly, this invention lies in the field of linings for large diameter boreholes, which are made of materials of low flexural rigidity and have means for locking to the grouting which ties them to the wall of the borehole.
2. Description of the Prior Art
Drilled shafts or boreholes in the earth are used for a wide variety of purposes, including entries into mines, points of entry for mine storage facilities and for the storage of biological waste and for other purposes.
The most common linings for drilled boreholes or shafts in the earth are rigid steel casings, which are grouted into place by filling the annulus between the borehole wall and the steel casing with a Portland cement grout. The cost of rigid steel casing is high, especially in cases where large diameters are involved and large hydrostatic pressures are anticipated. The steel casings have great weight, which necessitates the use of specialized equipment, capable of handling very large weights, to run the casing into the shaft or borehole.
Another disadvantage of grouted-in-place rigid steel casing is that of transporting the casing to the borehole, because of its size and weight.
Still another disadvantage of the rigid steel casing is the time involved in placing it into the shaft prior to grouting, because of the requirement that many joints must be welded together and the welds allowed to cool, before the casing can be further lowered into the hole. This is a stepwise operation, where one joint of casing is welded to a preceding joint, the weld is allowed to cool and then the casing is lowered by the length of the new joint prior to welding still another joint, etc.
A thin lightweight lining material offers many advantages in that the lining can in some application be an essentially continuous membrane, which will have a relatively low weight and can be delivered to the site in a collapsed condition. It may be delivered in one continuous length or in a few pieces of substantial length, requiring fewer joints to provide the full depth of liner.
Though the concept appears ideal, heretofore there has been no means of maintaining the correct internal diameter of membrane linings of low flexural rigidity while grouting in place or when subjected to external pressure.
This invention describes a new technique for using a combination of a membrane liner of low flexural rigidity with Portland cement grout to create a rigid waterproof lining which is impervious to the movement of the fluid, either into or out of the borehole, or shaft, and in which the membrane is rigidly joined to the grout and can withstand relatively high differential pressures by virtue of the reinforcement created by the enclosing cement grout.